this is for an sks but im looking for more atm, http://www.cheaperthandirt.com/SKS525-31.html
looks good and simple to use, but idk if this one will fit the m4 stocks. i know ther are some out there for this but i gotta find them

this is a replacement for your existing stock, but just goes over the "buffer tube" of your original stock, which should mean you just take the nut off the adjusting peice then pull the piece that locks down the stock out, then put this one on then replace the nut. http://www.cheaperthandirt.com/ARR189-7.html